Urban ruination is an understudied feature in the life of cities. This article discusses its causes. Based on the study of four shrinking Portuguese cities (Lisbon, Barreiro, Guimarães and Vizela), and using Multiple Linear Regression Analysis as the statistical method, the structure of relationships among ruins, economic change,demographic change, social geography and the characteristics of buildings are discussed. Although the study concludes that ruination is a highly contingent phenomenon, the results show that of all the structural factors,demographic ageing and the obsolescence of buildings (poor housing conditions) are the key causes of ruination in the four cities under study. Links between ruination and socio-spatial processes have also been identified.